1. Field of the Invention
The present invention relates to a master disk used for manufacturing an optical disk on which information is recorded, and a method for manufacturing the same.
2. Description of the Related Art
An optical disk such as a compact disk (CD) or a digital versatile disk (DVD) used as a medium for recording and/or reproducing video information, as shown in FIGS. 1A and 1B, includes lands 2 each having information pits 2a, and grooves 3 positioned between neighboring lands 2 for defining the lands 2. Recently, much attention has been paid to increasing the capacity of recordable information by forming information pits 3a in the grooves 3 of an optical disk 1.
To manufacture the optical disk 1, a master disk having a recording plane of the same pattern as the optical disk 1 is generally necessary. In other words, as shown in FIG. 2, a master disk 10 having lands 31, grooves 33 and information pits 32 and 34, which is of the same pattern as the optical disk 1, is necessary. To manufacture the master disk 10, first, a photoresist capable of being photochemically reacted with a laser beam having a predetermined wavelength is coated on a carefully polished glass substrate 20 to a predetermined thickness, to form a photoresist layer 30. While the substrate 20 is rotated, two laser beams are simultaneously incident onto the substrate 20, so that the portions of the photoresist layer 30, where the grooves 33 and the information pits 32 and 34 are to be formed, are exposed to the laser beams. Then, a photochemical reaction occurs in the portions exposed to the laser beams. During the exposing process, one laser beam is deflected to the left and right to expose the portion in which the grooves 33 are to be formed, and the other laser beam exposes the portion where the information pits 32 and 34 are to be formed. Then, the exposed portions are etched and cleaned, thereby finally manufacturing the master disk 10 as shown in FIG. 2.
Thereafter, a metal film (not shown) is deposited thinly on the recording plane of the master disk 10 and a metal such as nickel is electrically coated on the metal film to then separate the coated film from the master disk 10. The coated film is used as a stamper in the plastic molding of the optical disk 1. Such a stamper has surface irregularities exactly opposite to those of the recording plane of the master disk 10. Accordingly, the optical disk manufactured by using the stamper has the lands, grooves and information pits having the same pattern as the master disk.
However, if the master disk is manufactured in the above-described manner, grooves and information pits are both formed in the same photoresist layer. Thus, the boundary between the grooves and information pits becomes ambiguous, and the angles of the side walls of the groove and information pits with respect to the substrate become lower. Therefore, in the optical disk manufactured using the master disk, the boundary between the groove and information pit becomes ambiguous and the side walls of the grooves and information pits have a low angle. As a result, it is difficult to reproduce the information recorded on the optical disk by a recording/reproducing apparatus, using light diffraction from the boundary between the groove and information pit. Accordingly, the reproduction characteristics of the optical disk are deteriorated. Such a problem becomes serious in a high-definition (HD)-DVD having information tracks at a smaller pitch than the conventional DVD.